1. Field of the Invention
The present invention generally relates to a spurious signal reduction circuit, and more particularly to a spurious signal reduction circuit which is provided in a main signal circuit in a high-frequency radio transmitter and receiver for a radio communication apparatus, such as a mobile station, a base station and a multiplex station, and can prevent a spurious signal from radiating and mixing.
2. Description of the Related Art
FIG. 1 shows a high-frequency radio part in a conventional radio communication apparatus. In a high-frequency transmitter 10 in the high-frequency radio part, a transmitting baseband signal is modulated by a modulator (MOD) 11 using, for example, a QPSK method into an IF signal. Next, the IF signal is amplified by an IF amplifier (IFA) 12 and a predetermined band-limited IF signal is extracted by an IF filter (IFF) 13. Then, the predetermined band-limited IF signal is up-converted to an RF signal by an RF mixer 14 and a predetermined band-limited RF signal ft is extracted by an RF filter (RFF) 15. Finally, the predetermined band-limited RF signal ft is power-amplified by a high power amplifier (HPA) 16 and transmitted by an antenna 41 through an antenna common part 42.
A received RF signal from the antenna 41 is supplied to a high-frequency receiver 20 in the high-frequency radio part through the antenna common part 42. The received RF signal is amplified by a low-noise amplifier (LNA) 21 and a predetermined band-limited RF signal fr is extracted by an RF filter (RFF) 22. Then, the predetermined band-limited RF signal is down-converted to an IF signal by an RF mixer 23 and a predetermined band-limited IF signal is extracted by IF filters (IFF) 24 and 26. An amplitude of the IF signal is controlled to be a constant value by a feedback loop which comprises IF amplifiers (IFA) 25, 27 and an automatic gain control circuit (AGC) 29. Finally, the IF signal is demodulated by a demodulator (DEM) 28 using the QPSK method into a received base-band signal.
Generally, a radio communication apparatus comprises a plurality of local oscillators and mixers in the same case. Therefore, a spurious signal fs is generated based on a cross-modulation distortion among output signals of the local oscillators and the mixers. To suppress the spurious signal fs, the high-frequency transmitter and receiver are assembled in the form of separate modules. If possible, it is desirable that each RF part and IF part is assembled in the form of a separate module. Then, each module is sufficiently shielded electrically and magnetically and the RF part and the IF part are connected by means of a coaxial cable for a main signal. However, it is basically desirable to suppress a spurious signal at its source or a mixing point.
Conventionally, in the high-frequency transmitter 10 shown in FIG. 1, the RF filter (RFF) 15 is provided between the RF mixer 14 and the HPA 16 to extract the predetermined band-limited RF signal ft and to suppress a radiation of the spurious signal fs. In the high-frequency receiver 20 shown in FIG. 1, the RF filter (RFF) 22 is provided between the LNA 21 and the RF mixer 23 to extract the predetermined band-limited RF signal fr and to prevent the RF signal fr from mixing with the spurious signal fs.
FIG. 2 shows a conventional RF filter. A construction of the RF filter in the high-frequency receiver is the same construction as in the high-frequency transmitter. FIG. 2(A) shows an example of a construction of the RF filter using concentrated constant elements. A band-pass filter is constructed by an LC series circuit comprising an inductor L1 and a capacitor C1 and an LC parallel circuit comprising an inductor L2 and a capacitor C2. A 2nd-order band-pass filter which has a cascade of two band-pass filters is inserted between an input terminal IN and an output terminal OUT of a main signal line. Therefore, only a main signal having a predetermined bandwidth is passed and a spurious RF signal having frequencies other than a passband of the band-pass filter is stopped.
FIG. 2(B) shows an example of a construction of the RF filter using distributed constant elements. Strip lines 54, 55 each having a length equal to a half wavelength xcex/2 of a main signal are placed between an input terminal IN and an output terminal OUT on a dielectric substrate made of GaAs or molten silica, etc., and these components are connected electromagnetically through edges of these components. The strip lines 54, 55 operate as a resonator at the main signal frequency and they construct a band-pass filter. Therefore, only the main signal having a predetermined bandwidth is passed and a spurious RF signal having frequencies other than a pass-band of the bandpass filter is stopped.
However, when the LC series or parallel circuits are inserted in the main signal line shown in FIG. 2(A), a loss of the main signal is increased because resistance components and conductance components are substantially inserted in the main signal line as well as the inductors L1 and L2.
On the other hand, when the resonance lines 54, 55 each having the length xcex/2 are placed as shown in FIG. 2(B), a loss of the main signal is increased due to a conductor loss xcex1C, a dielectric loss xcex1D and a radiation loss xcex1R. Especially, over a frequency range between 30 GHz to 40 GHz, the radiation loss xcex1R from the resonance lines 54, 55 can not be ignored and this causes a reduction of a Q value. As a result, the strip lines 54, 55 can not operate as a resonator.
Therefore, in the conventional high-frequency radio apparatus, for a transmitter, a loss of the main signal and power consumption are large and a gain is reduced. On the other hand, for a receiver, a noise figure and a gain are reduced.
It is a general object of the present invention to provide a spurious signal reduction circuit in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a spurious signal reduction circuit which can prevent a spurious signal from radiating and mixing without a loss of a main signal.
The above objects of the present invention are achieved by a spurious signal reduction circuit which is connected to a main signal line. The spurious signal reduction circuit comprises a resistor having a terminal connected to the main signal line, an LC parallel resonance circuit and an LC series resonance circuit. The parallel resonance circuit has a resonance frequency which is equal to a main signal frequency and has a terminal connected to another terminal of the resistor. The series resonance circuit has a resonance frequency which is equal to a spurious signal frequency and has a terminal connected to the other terminal of the resistor.